Electronic Device Display with a Backlight

ABSTRACT

An electronic device may include display layers such as liquid crystal display layers and a backlight unit that provides illumination for the display layers. The backlight unit may include light-emitting diodes that emit light into the edge of a light guide film. To minimize the inactive area of the display, the light-emitting diodes may be tightly spaced to approximate a line light source instead of point light sources. Color and/or luminance compensation layers may be incorporated at various locations within the backlight structures to ensure that the backlight provided to the display layers is homogenous. A thin-film transistor layer of the display may be coupled to a printed circuit board by a flexible printed circuit. The flexible printed circuit may have additional solder mask layers to improve robustness, may include encapsulation, and may have traces with a varying pitch.

This application claims the benefit of provisional patent applicationNo. 62/751,438, filed Oct. 26, 2018, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to electronic devices with displays, and, moreparticularly, to displays with backlights.

Electronic devices such as computers, cellular telephones, and tabletshave displays. Some displays such as organic light-emitting diodedisplays have arrays of pixels that generate light. In displays of thistype, backlighting is not necessary because the pixels themselvesproduce light. Other displays contain passive pixels that can alter theamount of light that is transmitted through the display to displayinformation for a user. Passive pixels do not produce light themselves,so it is often desirable to provide backlight for a display with passivepixels. Passive pixels may be formed from a layer of liquid crystalmaterial formed between two electrode layers and two polarizer layers.

In a typical backlight assembly for a display, a light guide plate isused to distribute backlight generated by a light source such as alight-emitting diode light source. A reflector may be formed under thelight guide plate to improve backlight efficiency.

Conventional backlight assemblies may cause visible artifacts, may notbe robust, and may occupy an undesirably large amount of space within anelectronic device.

It would therefore be desirable to be able to provide displays withimproved backlights.

SUMMARY

A display may have an array of pixels for displaying images for aviewer. The array of pixels may be formed from display layers such as acolor filter layer, a liquid crystal layer, a thin-film transistorlayer, an upper polarizer layer, and a lower polarizer layer.

A backlight unit may be used to produce backlight illumination for thedisplay. The backlight illumination may pass through the polarizers, thethin-film transistor layer, the liquid crystal layer, and the colorfilter layer. The backlight unit may have a row of light-emitting diodesthat are mounted on a flexible printed circuit board and that emit lightinto a light guide layer.

To minimize the size of the inactive area of the display, thelight-emitting diodes that provide the backlight to the light guidelayer may be positioned close together along the flexible printedcircuit board. The light-emitting diodes may be flip chip bonded to theflexible printed circuit board and may approximate a line light sourceinstead of separate point light sources.

Color and/or luminance compensation layers may be incorporated atvarious locations within the backlight structures to ensure that thebacklight provided to the display layers is homogenous. For example,colored tape may be included on light guide layer tabs or a coloredadhesive layer may be used to attach the light guide layer to a chassis.

The thin-film transistor layer of the display may be coupled to aprinted circuit board by a flexible printed circuit. The display driverintegrated circuit for the thin-film transistor layer may be mounted onthe flexible printed circuit. The flexible printed circuit may haveadditional solder mask layers to improve the robustness of the flexibleprinted circuit. The flexible printed circuit may also includeencapsulation. The flexible printed circuit may have traces with a pitchthat varies across the length of the flexible printed circuit. Astainless steel stiffener may be included on the flexible printedcircuit opposite the display driver integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative electronic devicehaving a display in accordance with an embodiment.

FIG. 2 is a cross-sectional side view of an illustrative display in anelectronic device in accordance with an embodiment.

FIG. 3 is a top view of an illustrative display in accordance with anembodiment.

FIG. 4 is a cross-sectional side view of illustrative backlightstructures showing the edge of a light guide film adjacent to a plasticchassis in accordance with an embodiment.

FIG. 5 is a cross-sectional side view of illustrative backlightstructures showing a light-emitting diode that emits light into the edgeof a light guide film in accordance with an embodiment.

FIG. 6 is a top view of illustrative backlight structures showinglight-emitting diode packages that each include first and secondlight-emitting diodes in accordance with an embodiment.

FIG. 7 is a cross-sectional side view of an illustrative light-emittingdiode package that includes first and second light-emitting diodes andthat is flip chip bonded to a flexible printed circuit in accordancewith an embodiment.

FIG. 8 is a circuit diagram of an illustrative light-emitting diodepackage that includes first and second light-emitting diodes inaccordance with an embodiment.

FIG. 9 is a top view of an illustrative light guide layer that includeslight-scattering features that extend across the light guide layer inaccordance with an embodiment.

FIG. 10 is a cross-sectional side view of illustrative light-scatteringfeatures having a cross-sectional shape of a right triangle with aninety degree angle in accordance with an embodiment.

FIG. 11 is a cross-sectional side view of illustrative light-scatteringfeatures having a cross-sectional shape of a triangle with an anglegreater than ninety degrees in accordance with an embodiment.

FIG. 12 is a cross-sectional side view of illustrative light-scatteringfeatures having a cross-sectional shape with a curved upper surface inaccordance with an embodiment.

FIG. 13 is a cross-sectional side view of illustrative light-scatteringfeatures having alternating cross-sectional shapes with differentheights in accordance with an embodiment.

FIG. 14 is a cross-sectional side view of illustrative light-scatteringfeatures having a cross-sectional shape of a triangle with a rounded tipin accordance with an embodiment.

FIG. 15 is a top view of an illustrative turning film that includesprisms that extend across the turning film in accordance with anembodiment.

FIG. 16 is a cross-sectional side view of illustrative prims having across-sectional shape with one planar surface and one curved surface inaccordance with an embodiment.

FIG. 17 is a cross-sectional side view of illustrative prims having across-sectional shape with two curved surfaces having the same curvaturein accordance with an embodiment.

FIG. 18 is a cross-sectional side view of illustrative prims having across-sectional shape with two curved surfaces having differentcurvature in accordance with an embodiment.

FIG. 19 is a cross-sectional side view of illustrative prims having across-sectional shape with a curved surface in accordance with anembodiment.

FIG. 20 is a cross-sectional side view of illustrative backlightstructures including additional ink or colored layers to correct colorand/or luminance variations in the emitted backlight in accordance withan embodiment.

FIG. 21 is a top view of an illustrative light guide layer having tabsin accordance with an embodiment.

FIG. 22 is a cross-sectional side view of an illustrative light guidelayer having tabs and a color and/or luminance correcting layer mountedover the tab in accordance with an embodiment.

FIG. 23 is a top view of illustrative display layers showing how thedisplay layers may have a non-uniformity such as a bright band along theupper edge when in accordance with an embodiment.

FIGS. 24 and 25 are top views of illustrative backlight structuresshowing how ink may be patterned above the light guide layer tocompensate for a non-uniformity of the display in accordance with anembodiment.

FIG. 26 is a cross-sectional side view of illustrative backlightstructures showing how ink may be patterned above the light guide layerto compensate for a non-uniformity of the display in accordance with anembodiment.

FIG. 27 is a cross-sectional side view of illustrative backlightstructures that include an additional optical film between a turningfilm and display layers in accordance with an embodiment.

FIG. 28 is a cross-sectional side view of the illustrative additionaloptical film between the turning film and the display layers inaccordance with an embodiment.

FIG. 29 is a cross-sectional side view of the illustrative additionaloptical film between the turning film and the display layers with adiffusive pressure sensitive adhesive layer in accordance with anembodiment.

FIG. 30 is a diagram of an illustrative system used to capture images ofthe display of an electronic device for compensation in accordance withan embodiment.

FIG. 31 is a diagram of illustrative method steps used to determinecompensation values for each pixel in a display in accordance with anembodiment.

FIG. 32 is a diagram of an illustrative display showing how thecompensation values for each pixel may be used to modify image databefore the image data is provided to display driver circuitry inaccordance with an embodiment.

FIG. 33 is a cross-sectional side view of an illustrative display with aflexible printed circuit that attaches a thin-film transistor layer to arigid printed circuit board in accordance with an embodiment.

FIG. 34 is a top view of an illustrative flexible printed circuit thatattaches a thin-film transistor layer to a rigid printed circuit boardand that has a stiffener in accordance with an embodiment.

FIG. 35 is a cross-sectional side view of an illustrative attachmentregion between a flexible printed circuit and a rigid printed circuitboard in accordance with an embodiment.

FIG. 36 is a top view of an illustrative attachment region between aflexible printed circuit and a rigid printed circuit board showingisland-shaped gap-filling layers in accordance with an embodiment.

FIG. 37 is a top view of an illustrative flexible printed circuit havingtraces with a pitch that varies across the flexible printed circuit inaccordance with an embodiment.

FIG. 38 is a graph showing illustrative profiles of the pitch anddensity of the flexible printed circuit traces as a function of positionin accordance with an embodiment.

FIG. 39 is a graph showing illustrative tolerance and pitch profiles ofthe flexible printed circuit traces as a function of position inaccordance with an embodiment.

FIG. 40 is a cross-sectional side view of an illustrative attachmentregion between a flexible printed circuit and a thin-film transistorlayer in accordance with an embodiment.

FIG. 41 is a cross-sectional side view of an illustrative flexibleprinted circuit in which a tin layer has been omitted in a bendingregion in accordance with an embodiment.

FIG. 42 is a cross-sectional side view of an illustrative flexibleprinted circuit in which a tin layer has been omitted in a bendingregion and only one solder resist is included in the bending region inaccordance with an embodiment.

FIGS. 43A and 43B shows illustrative tables of RGB values associatedwith different white points in accordance with an embodiment.

FIG. 44 is a graph showing how red, blue, and green sub-pixel brightnesslevels may vary during white-to-black transitions in accordance with anembodiment.

FIG. 45 shows how pixels may be provided with intermediate target valuesduring transitions to mitigate visible artifacts during the transitionsin accordance with an embodiment.

FIG. 46 is a diagram of illustrative resources that may be used in anelectronic device to reduce color-artifacts such as a green appearanceduring white-to-black transitions in accordance with an embodiment.

FIG. 47 is a flowchart of illustrative operations involved in usingresources of the type shown in FIG. 46 in displaying content withreduced artifacts in accordance with an embodiment.

DETAILED DESCRIPTION

An illustrative electronic device of the type that may be provided witha display is shown in FIG. 1. As shown in FIG. 1, electronic device 10may have control circuitry 16. Control circuitry 16 may include storageand processing circuitry for supporting the operation of device 10. Thestorage and processing circuitry may include storage such as hard diskdrive storage, nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory configured to form a solidstate drive), volatile memory (e.g., static or dynamicrandom-access-memory), etc. Processing circuitry in control circuitry 16may be used to control the operation of device 10. The processingcircuitry may be based on one or more microprocessors, microcontrollers,digital signal processors, baseband processors, power management units,audio chips, application specific integrated circuits, etc.

Input-output circuitry in device 10 such as input-output devices 12 maybe used to allow data to be supplied to device 10 and to allow data tobe provided from device 10 to external devices. Input-output devices 12may include buttons, joysticks, scrolling wheels, touch pads, key pads,keyboards, microphones, speakers, tone generators, vibrators, cameras,sensors, light-emitting diodes and other status indicators, data ports,etc. A user can control the operation of device 10 by supplying commandsthrough input-output devices 12 and may receive status information andother output from device 10 using the output resources of input-outputdevices 12.

Input-output devices 12 may include one or more displays such as display14. Display 14 may be a touch screen display that includes a touchsensor for gathering touch input from a user or display 14 may beinsensitive to touch. A touch sensor for display 14 may be based on anarray of capacitive touch sensor electrodes, acoustic touch sensorstructures, resistive touch components, force-based touch sensorstructures, a light-based touch sensor, or other suitable touch sensorarrangements.

Control circuitry 16 may be used to run software on device 10 such asoperating system code and applications. During operation of device 10,the software running on control circuitry 16 may display images ondisplay 14.

Device 10 may be a tablet computer, laptop computer, a desktop computer,a television, a cellular telephone, a media player, a wristwatch deviceor other wearable electronic equipment, or other suitable electronicdevice.

Display 14 for device 10 includes an array of pixels. The array ofpixels may be formed from liquid crystal display (LCD) components orother suitable display structures. Configurations based on liquidcrystal display structures are sometimes described herein as an example.

A display cover layer may cover the surface of display 14 or a displaylayer such as a color filter layer, thin-film transistor layer, or otherportion of a display may be used as the outermost (or nearly outermost)layer in display 14. The outermost display layer may be formed from atransparent glass sheet, a clear plastic layer, or other transparentmember.

A cross-sectional side view of an illustrative configuration for display14 of device 10 is shown in FIG. 2. As shown in FIG. 2, display 14 mayinclude a backlight unit such as backlight unit 42 (sometimes referredto as a backlight or backlight structures) for producing backlight 44.During operation, backlight 44 travels outwards (vertically upwards indimension Z in the orientation of FIG. 2) and passes through pixelstructures in display layers 46. This illuminates any images that arebeing produced by the pixels for viewing by a user. For example,backlight 44 may illuminate images on display layers 46 that are beingviewed by viewer 48 in direction 50.

Display layers 46 may be mounted in chassis structures such as a plasticchassis structure and/or a metal chassis structure to form a displaymodule for mounting in a housing in device 10 or display layers 46 maybe mounted directly in an electronic device housing for device 10 (e.g.,by stacking display layers 46 into a recessed portion in a metal orplastic housing). Display layers 46 may form a liquid crystal display ormay be used in forming displays of other types.

In a configuration in which display layers 46 are used in forming aliquid crystal display, display layers 46 may include a liquid crystallayer such a liquid crystal layer 52. Liquid crystal layer 52 may besandwiched between display layers such as display layers 58 and 56.Layers 56 and 58 may be interposed between lower polarizer layer 60 andupper polarizer layer 54.

Layers 58 and 56 may be formed from transparent substrate layers such asclear layers of glass or plastic. Layers 56 and 58 may be layers such asa thin-film transistor layer and/or a color filter layer. Conductivetraces, color filter elements, transistors, and other circuits andstructures may be formed on the substrates of layers 58 and 56 (e.g., toform a thin-film transistor layer and/or a color filter layer). Touchsensor electrodes may also be incorporated into layers such as layers 58and 56 and/or touch sensor electrodes may be formed on other substrates.

With one illustrative configuration, layer 58 may be a thin-filmtransistor layer that includes an array of pixel circuits based onthin-film transistors and associated electrodes (pixel electrodes) forapplying electric fields to liquid crystal layer 52 and therebydisplaying images on display 14. Layer 56 may be a color filter layerthat includes an array of color filter elements for providing display 14with the ability to display color images. If desired, layer 58 may be acolor filter layer and layer 56 may be a thin-film transistor layer.Configurations in which color filter elements are combined withthin-film transistor structures on a common substrate layer may also beused.

During operation of display 14 in device 10, control circuitry (e.g.,one or more integrated circuits on a printed circuit) may be used togenerate information to be displayed on display 14 (e.g., display data).The information to be displayed may be conveyed to a display driverintegrated circuit such as circuit 62A or 62B using a signal path suchas a signal path formed from conductive metal traces in a rigid orflexible printed circuit such as printed circuit 64 (as an example).Integrated circuits such as integrated circuit 62A and/or flexibleprinted circuits such as flexible printed circuit 64 may be attached tosubstrate 58 in ledge region 66 (as an example).

Backlight structures 42 may include a light guide layer such as lightguide layer 78. Light guide layer 78 may be formed from a transparentmaterial such as clear glass or plastic. During operation of backlightstructures 42, a light source such as light source 72 may generate light74. Light source 72 may be, for example, an array of light-emittingdiodes (e.g., a series of light-emitting diodes that are arranged in arow that extends into the page in the orientation of FIG. 2). The arrayof light-emitting diodes may be mounted to a rigid or flexible printedcircuit. The printed circuit may be adhered to adjacent layers in theelectronic device. In certain embodiments, the printed circuit may beadhered to portions of light guide layer 78.

Light 74 from light source 72 may be coupled into edge surface 76 oflight guide layer 78 and may be distributed in dimensions X and Ythroughout light guide layer 78 due to the principal of total internalreflection. Light guide layer 78 may include light-scattering featuressuch as pits, bumps, grooves, or ridges that help light exit light guidelayer 78 for use as backlight 44. These features may be located on anupper surface and/or on an opposing lower surface of light guide layer78. With one illustrative configuration, a first surface such as thelower surface of light guide layer 78 has a pattern of bumps and anopposing second surface such as the upper surface of light guide layer78 has a pattern of ridges (sometimes referred to as lenticules,lenticular structures, or lenticular ridges). Light source 72 may belocated at any desired edge of light guide layer 78.

Light 74 that scatters upwards in direction Z from light guide layer 78may serve as backlight 44 for display 14. Light 74 that scattersdownwards may be reflected back in the upward direction by reflector 80.Reflector 80 may be formed from a reflective structure such as asubstrate layer of plastic coated with a dielectric mirror formed fromalternating high-index-of-refraction and low-index-of-refractioninorganic or organic layers. Reflector 80 may be formed from areflective material such as a layer of white plastic or other shinymaterials.

To enhance backlight performance for backlight structures 42, backlightstructures 42 may include optical films 70. Optical films 70 may includediffuser layers for helping to homogenize backlight 44 and therebyreduce hotspots. Optical films 70 may also include brightnessenhancement films for collimating backlight 44. Optical films 70 mayoverlap the other structures in backlight unit 42 such as light guidelayer 78 and reflector 80. For example, if light guide layer 78 has arectangular footprint in the X-Y plane of FIG. 2, optical films 70 andreflector 80 may each have a matching rectangular footprint. Opticalfilms 70 may include compensation films for enhancing off-axis viewingor compensation films may be formed within the polarizer layers ofdisplay 14 or elsewhere in display 14.

FIG. 3 is a top view of a portion of display 14 showing how display 14may have an array of pixels 90 formed within display layers 46. Pixels90 may have color filter elements of different colors such as red colorfilter elements, green color filter elements, and blue color filterelements. Pixels 90 may be arranged in rows and columns and may formactive area AA of display 14. The borders of active area AA may beslightly inboard of the borders of light-guide layer 78 to ensure thatthere are no visible hotspots in display 14 (i.e., no areas in which thebacklight illumination for display 14 is noticeably brighter thansurrounding areas). For example, border 92 of active area AA may beoffset by a distance 82 from left edge 76 of light guide layer 78.

It is generally desirable to minimize the size of distance 82 so thatdisplay 14 is as compact as possible for a given active area size. Thisminimizes the size of the inactive area (IA) of the display.Nevertheless, distance 82 should not be too small to ensure that thereis adequate light mixing. In particular, distance 82 should besufficiently large to allow light 74 that is emitted from light-emittingdiodes 72 to homogenize enough to serve as backlight illumination.Distance 82 is often as long as necessary to ensure light fromlight-emitting diodes 72 is sufficiently mixed. Accordingly, distance 82may sometimes be referred to as mixing distance 82. When light 74 isinitially emitted from individual light-emitting diodes 72, light 74 isconcentrated at the exits of light-emitting diodes 72 and is absent inthe spaces between light-emitting diodes 72. After light 74 haspropagated sufficiently far within light-guide plate 78 (i.e., afterlight 74 has traversed a sufficiently large mixing distance 82), light74 will be smoothly distributed along dimension X and will no longer beconcentrated near the exits of respective individual light-emittingdiodes 72. To minimize mixing distance 82, light-emitting diodes 72 maybe positioned close together such that the light-emitting diodesapproximate a line source instead of separate point sources.

The rectangular shape of light guide layer 78 and active area AA in FIG.3 is merely illustrative. If desired, light guide layer 78 and/or theactive area AA may have a non-rectangular shape (e.g., a shape with oneor more curved portions). For example, the active area and/or lightguide layer may have a rectangular outline with rounded corners. Theactive area and/or light guide layer may have a notch along an upperedge that accommodates additional components such as sensors. The activearea and light guide layer may have different shapes if desired. Forexample, the light guide layer may have a rectangular shape withright-angled corners (as shown in FIG. 3) whereas the active area mayhave a rectangular shape with rounded corners.

Detailed cross-sections of backlight structures 42 are shown in FIGS. 4and 5. In particular, FIG. 4 is a cross-sectional side view of backlightstructures 42 taken along line 98 in FIG. 3. FIG. 5 is a cross-sectionalside view of backlight structures 42 taken along line 97 in FIG. 3.

As shown in FIG. 4, backlight structures 42 include light guide layer 78and a reflector 80 attached to a lower surface of the light guide layer.Optical film 70-1 may be placed on an upper surface of the light guidelayer. Optical film 70-1 may be a turning film that directs light fromthe light guide layer vertically towards the overlying display layers.The optical film may be matte and therefore may sometimes be referred toas matte turning film 70-1. Any desired additional optical films may beincorporated above turning film 70-1. For example, a diffuser layer maybe incorporated on an upper surface of turning film 70-1 (or positionedabove 70-1 and separated from the upper surface of the turning film). Ingeneral, any desired additional optical films may be incorporated aboveoptical film 70-1 in any desired locations (e.g., positioned directly onthe underlying optical film or separated from the underlying opticalfilm by a gap). Turning film 70-1 may help homogenize the backlight andensure that the light is directed vertically towards the viewer. Theturning film may also be used to control the viewing angle of thedisplay.

Backlight structures 42 also include chassis 102. Chassis 102 may be aplastic chassis (sometimes referred to as a p-chassis) that supportsother layers (e.g., layers in backlight structures 42 and/or displaylayers 46) in the display. Chassis 102 may extend around the peripheryof light guide layer 78 with a central opening in which the light guidelayer 78 is positioned (e.g., chassis 102 may be ring-shaped). Ifdesired, chassis 102 may be formed from two or more types of material.For example, chassis 102 may be formed from two or more shots of moldedplastic having different colors. This example is merely illustrative andchassis 102 may be formed from a single dielectric material if desired.

Backlight structures 42 may include an additional chassis 103. Chassis103 may be a metal chassis (sometimes referred to as an m-chassis) thatsupports other layers (e.g., layers in backlight structures 42 and/ordisplay layers 46) in the display. Chassis 103 may extend under theentire light guide layer 78 if desired (instead of having a ring-shapelike chassis 102). In an alternate embodiment, however, chassis 103 mayalso be ring-shaped. An additional adhesive layer 107 may be interposedbetween an edge of light guide layer 78 and metal chassis 103.

Adhesive layer 104 may be attached to a top surface of chassis 102.Adhesive layer 104 may optionally be attached to a top surface ofoptical film 70-1. Adhesive layer 104 may extend around the periphery oflight guide layer 78 and may have a central opening (e.g., adhesivelayer 104 may be ring-shaped). Adhesive layer 104 may thereforesometimes be referred to as ring tape. Ring tape 104 may attachbacklight structures 42 to display layers 46 if desired. Ring tape mayalternatively attach chassis 102 to other layers (e.g., optical films)within backlight structures 42. Another adhesive layer 106 may attach alower surface of chassis 102 to an upper surface of chassis 103.Adhesive layers 104 and 106 may be pressure sensitive adhesive layers orany other desired type of adhesive layers.

To minimize the width of the inactive area of the display, the distance206 between chassis 102 and light guide layer 72 may be minimized. Forexample, distance 206 may be less than 1 millimeter, less than 0.5millimeters, less than 0.4 millimeters, less than 0.3 millimeters,between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters,between 0.25 and 0.35 millimeters, greater than 0.2 millimeters, etc.

The three edges of backlight structures 42 that do not includelight-emitting diodes 72 may have cross-sections of the type shown inFIG. 4. For example, looking at FIG. 3, light-emitting diodes 72 arepositioned along a left edge of the light guide layer. The left edge ofthe backlight structures has a different arrangement than the remainingthree edges of the backlight structures. The remaining three edges ofthe backlight structures may have an arrangement of the type shown inFIG. 4. The example of the light-emitting diodes being positioned alongthe left edge of the light guide layer is merely illustrative. Thelight-emitting diodes may be positioned along any edge (or more than oneedge) of the light guide layer.

FIG. 5 is a cross-sectional side view of the left edge of the backlightstructures where the light sources emit light into the light guidelayer. As shown in FIG. 5, this portion of the backlight structuresstill includes light guide layer 78, a reflector layer 80 attached to alower surface of the light guide layer, and an optical film such asturning film 70-1 attached to an upper surface of the light guide layer.However, light sources 72 may be included to emit light 74 in theY-direction through edge surface 76 of the light guide layer.

Light sources 72 may be light-emitting diodes that are arranged in a rowalong the edge surface 76 of the light guide layer. Each light-emittingdiode 72 may be mounted on a printed circuit board such as flexibleprinted circuit board 108. Flexible printed circuit board 108 may be aprinted circuit formed from sheets of polyimide or other flexiblepolymer layers. Flexible printed circuit board 108 (sometimes referredto as flexible printed circuit 108 or printed circuit 108) may includepatterned metal traces for carrying signals between components on theflexible printed circuit board. Flexible printed circuit board 108 mayoptionally include contact pads (e.g., solder pads) on an upper surfaceof the flexible printed circuit board. Solder may be used to couple thelight-emitting diodes to solder pads on the flexible printed circuitboard. Each light-emitting diode (sometimes referred to as alight-emitting diode package) may have one or more associated solderpads. The solder may electrically and mechanically connect thelight-emitting diodes to flexible printed circuit board. Optionally, anadditional adhesive layer may be attached to both the upper surface ofthe light-emitting diodes 72 and the upper surface of light guide layer78.

Flexible printed circuit board 108 may also be coupled to light guidelayer 78 by an additional adhesive layer. As shown in FIG. 5, flexibleprinted circuit board 108 may include an adhesive layer 116 that isattached between light guide layer 78 and flexible printed circuit 108.Adhesive layer 116 may be a pressure sensitive adhesive layer or anyother desired type of adhesive layer.

Ring tape 104 may be attached to an upper surface of chassis 103.Chassis 103 may be attached to a lower surface of flexible printedcircuit 108 by adhesive layer 130. Adhesive layer 130 may be a pressuresensitive adhesive layer or any other desired type of adhesive layer.Backlight structures 42 may also include one or more additional adhesivelayers that attach chassis 103 to reflector layer 80.

The light guide layer 78 of the backlight structures in FIGS. 4 and 5may be a light guide film (LGF) formed from a polymer material such aspolycarbonate. Using a polycarbonate light guide film may result in thelight guide layer having a small thickness 202 (see FIG. 0.5). Forexample, thickness 202 may be less than 1 millimeter, less than 0.5millimeters, less than 0.4 millimeters, less than 0.3 millimeters,between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters,between 0.35 and 0.40 millimeters, greater than 0.2 millimeters, etc.

Additionally, the arrangement of FIG. 5 minimizes the mixing distance 82between light-emitting diodes 72 and the active area of the display. Forexample, distance 82 may be less than 5.0 millimeters, less than 4.0millimeters, less than 3.5 millimeters, less than 3.0 millimeters, lessthan 2.0 millimeters, greater than 1.0 millimeter, between 2.0 and 4.0millimeters, between 2.5 and 3.5 millimeters, etc.

The small mixing distance in FIG. 5 may be a result of havinglight-emitting diodes 72 be positioned close together on flexibleprinted circuit 108. By positioning the light-emitting diodes closetogether, the light emitted from the light-emitting diodes will betterapproximate a uniform line light source rather than separate point lightsources.

FIG. 6 is a top view of backlight structures illustrating how thelight-emitting diode packages may be positioned close together tominimize mixing distance. As shown in FIG. 6, each light-emitting diodepackage 208 may include first and second light-emitting diodes 72-1 and72-2. The edge-to-edge distance between adjacent light-emitting diodepackages (e.g., distance 211) may be less than 1 millimeter, less than0.5 millimeters, less than 0.4 millimeters, less than 0.3 millimeters,between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters,between 0.25 and 0.35 millimeters, greater than 0.2 millimeters, etc.The pitch of the light-emitting diode packages 208 (e.g., thecenter-to-center distance 213 between adjacent light-emitting diodepackages) may be less than 5.0 millimeters, less than 4.0 millimeters,less than 3.5 millimeters, less than 3.0 millimeters, less than 2.0millimeters, greater than 1.0 millimeter, between 2.0 and 4.0millimeters, between 2.5 and 3.5 millimeters, between 3.0 and 4.0millimeters, between 3.0 and 3.5 millimeters, etc. The pitch of thelight-emitting diodes (e.g., the center-to-center distance 215 betweenadjacent light-emitting diodes) may be less than 4.0 millimeters, lessthan 3.0 millimeters, less than 2.0 millimeters, less than 1.5millimeters, less than 1.0 millimeters, greater than 1.0 millimeter,between 1.0 and 2.0 millimeters, between 1.0 and 2.5 millimeters,between 1.5 and 2.0 millimeters, etc. The edge-to-edge distance betweenadjacent light-emitting diodes (e.g., distance 217) may be less than 1millimeter, less than 0.8 millimeters, less than 0.6 millimeters, lessthan 0.4 millimeters, between 0.4 and 0.6 millimeters, between 0.45 and0.55 millimeters, greater than 0.3 millimeters, etc.

To form the light-emitting diode packages close together as in FIG. 6,the light-emitting diode packages may be flip chip bonded to theunderlying flexible printed circuit board (108). An example of this typeis shown in FIG. 7. As shown in FIG. 7, the light-emitting diode package208 may be attached to flexible printed circuit board 108 using first,second, and third flip chip bumps 210-1, 210-2, and 210-3. Each flipchip bump may be formed from solder and therefore may sometimes bereferred to as a solder ball.

Light-emitting diode package 208 may have three recesses 212 formed in afirst portion 208-1 of the light-emitting diode package. Each recess mayreceive a corresponding solder ball 210. The second portion 208-2 of thelight-emitting diode package may include first and second light-emittingdiodes 72-1 and 72-2. The light-emitting diodes 72-1 and 72-2 may becoupled in series between a corresponding anode (e.g., solder ball210-3) and cathode (e.g., solder all 210-1). The intervening solder ball210-2 may serve as a heat sink and may allow individual testing oflight-emitting diodes 72-1 and 72-2.

A solder pad may be included in each recess 212 to electrically connectthe light-emitting diode package to the solder balls. Flexible printedcircuit board 108 may have corresponding recesses 214, each of whichreceives a solder ball. A solder pad may be included in each recess 214to electrically connect the flexible printed circuit board to the solderballs. The example in FIG. 7 of both light-emitting diode package 208and flexible printed circuit board 108 having recesses to receive thesolder balls is merely illustrative. In general, recesses may be formedon none, one, or both of the light-emitting diode package 208 andflexible printed circuit board 108 for each solder ball.

The recesses 214 may have larger widths than their correspondingrecesses 212. Additionally, the recess 212 associated with solder ball210-2 may be wider than the recesses 212 associated with solder balls210-1 and 210-3. Similarly the recess 214 associated with solder ball210-2 may be wider than the recesses 214 associated with solder balls210-1 and 210-3.

FIG. 8 is a circuit diagram showing the circuit associated withlight-emitting diode package 208 of FIG. 7. As shown, light-emittingdiodes 72-1 and 72-2 are coupled in series between the anode and thecathode. Solder ball 210-1 may be coupled to the cathode voltage whereassolder ball 210-3 may be coupled to the anode voltage. Solder ball 210-2may be coupled to a node 218 between the light-emitting diodes 72-1 and72-2. A voltage for testing one of the light-emitting diodes mayoptionally be coupled to solder ball 210-2.

As previously mentioned, light guide layer 78 may have light-scatteringfeatures such as pits, bumps, grooves, or ridges that help light exitlight guide layer 78 for use as backlight 44. Light guide layer 78 maywork in combination with turning film 70-1 to ensure that backlight 44is provided to the display layers 46 in a desired direction at a desiredviewing angle. FIG. 9 is a top view of light guide layer 78 showing howthe light guide layer 78 may have light-scattering features 252(sometimes referred to as light-scattering structures) that extend alonga longitudinal axis orthogonal to the edge 76 of the light guide layerthat receives light from light-emitting diode packages 208.Light-scattering structures 252 extend parallel to the direction inwhich light is emitted from the light-emitting diodes. Light-scatteringfeatures 252 may extend entirely across the light guide layer or mayonly extend partially across the light guide layer. The density oflight-scattering structures 252 may be uniform across the light guidelayer or may vary across the light guide layer (e.g., may vary along theX-axis and/or Y-axis). Light-scattering structures 252 may sometimes bereferred to as lenticular light-scattering structures.

Light-scattering structures 252 may have any desired shape. FIG. 10 is across-sectional side view of a light guiding layer 78 havinglight-scattering structures 252 with a triangular cross-sectional shape.As shown in FIG. 10, the light-scattering structures are formed on anupper surface 75 of the light guide layer. The light-scatteringstructures may be formed from the same material as the light guide layer(and therefore may be formed integrally with the light guide layer) ormay be formed from a different material than the light guide layer.

The triangular light-scattering structures have first and secondsurfaces that meet at a vertex defined by angle θ₁. In FIG. 10, θ₁ isequal to 90 degrees (e.g., the triangular cross-section is a righttriangle). Light may reflect off of the surfaces of the light-scatteringstructure due to total internal reflection. As shown in FIG. 10, lightthat starts in the positive Z-direction will be reflected bylight-scattering structure 252 and follow path 254. When thelight-scattering structure has a triangular cross-section with aright-angle the light may not be mixed effectively. In FIG. 11, thelight-scattering structures are characterized by an angle θ₂ that isgreater than 90°. For example, θ₂ may be greater than 100°, greater than110°, greater than 120°, greater than 135°, between 91° and 110°,between 95° and 105°, greater than 95°, less than 145°, etc. Light thatstarts in the positive Z-direction will be reflected by light-scatteringstructure 252 and follow path 256. As shown in FIG. 11, when the angleθ₂ is greater than 90°, light may be mixed more than in FIG. 10 when theangle is equal to 90°.

FIG. 12 is a cross-sectional side view of a light guiding layer 78having light-scattering structures 252 with a semi-circularcross-sectional shape. As shown in FIG. 12, the light-scatteringstructures are formed on an upper surface 75 of the light guide layer.The light-scattering structures may be formed from the same material asthe light guide layer (and therefore may be formed integrally with thelight guide layer) or may be formed from a different material than thelight guide layer. The light-scattering structures have an upper surfacethat is curved. The upper surface may have a uniform radius of curvatureacross the cross-section of the light-scattering structure (e.g., as ina semi-circular cross-section). The upper surface may also have anon-uniform radius of curvature across the cross-section of thelight-scattering structure. The radius of curvature may vary along thelength of the light-scattering structure if desired.

The shape of the light-scattering structure in FIG. 12 may also mix thelight more effectively than the light-scattering structures in FIG. 10.Light that starts in the positive Z-direction will be reflected bylight-scattering structure 252 and follow path 258. Even when thelight-scattering structure has a curved upper surface, thelight-scattering structure may be characterized by an angle θ₃. Theangle θ₃ associated with the curved cross-sectional shape may be theangle of the vertex of a triangle drawn between the top-most point ofthe curved upper surface and the edges of the curved upper surface (atthe point they meet the planar upper surface 75 of the light guidelayer). The angle θ₃ may be greater than 90° to promote light mixing.For example, θ₃ may be greater than 100°, greater than 110°, greaterthan 120°, greater than 135°, between 91° and 110°, between 95° and105°, greater than 95°, less than 145°, etc.

If desired, the light-scattering structures 252 on the upper surface oflight guide layer 78 may vary in shape. As shown in FIG. 13,light-scattering structures 252-1 having a first shape may alternatewith light-scattering structures 252-2 having a second shape. The firstlight-scattering structures 252-1 may have a first height 260 that isgreater than the height 262 of the second light-scattering structures252-2. Alternating the light-scattering structures in this way mayimprove the light mixing within the light guide layer.

Both light-scattering structures 252-1 and light-scattering structures252-2 may have an associated angle θ. Light-scattering structures 252-1have an associated angle θ₁ whereas light scattering structures 252-2have an associated angle θ₂. In this example, light-scatteringstructures 252-1 and light-scattering structures 252-2 have the samewidth. Therefore, because height 260 is greater than height 262, angleθ₁ is less than angle θ₂. However, both angles θ₁ and θ₂ may be greaterthan 90°. The difference between angles θ₁ and θ₂ may be greater than5°, greater than 10°, greater than 20°, less than 25°, etc.

FIG. 14 shows yet another example for a cross-sectional shape oflight-scattering structures 252. As shown in FIG. 14, thelight-scattering structures may have a triangular shape with a roundedtip. Each light-scattering structure includes a planar surface 264, aplanar surface 266, and a curved surface 268 (sometimes referred to asrounded surface 268, rounded tip 268, rounded vertex 268, etc.)interposed between the planar surfaces 264 and 266. Having the roundedtip 268 instead of an angled vertex may improve light mixing. The angleθ associated with each light-scattering structure may be greater than90° to promote light mixing. For example, θ may be greater than 100°,greater than 110°, greater than 120°, greater than 135°, between 91° and110°, between 95° and 105°, greater than 95°, less than 145°, etc.

In general, each light-scattering structure may have any desiredstructure. The light-scattering structures may have differentstructures, the same structures, structures that vary in any desiredpattern, etc. The light-scattering structures may themselves have auniform cross-sectional shape or a cross-sectional shape that variesalong the length of the light-scattering structure. The light-scatteringstructures may be formed on the upper surface of the light guide layerand/or the lower surface of the light guide layer. Any of the potentiallight-scattering structure arrangements described herein may be used inthe light guide layer of FIGS. 4 and 5.

Turning film 70-1 (see FIGS. 4 and 5) may also have light-redirectingfeatures such as pits, bumps, grooves, ridges, or prisms that helpredirect light in a desired direction for use as backlight 44. FIG. 15is a top view of turning film 70-1 showing how the turning film 70-1 mayhave prisms 272 that extend along a longitudinal axis parallel to theedge 76 of the light guide layer that receives light from light-emittingdiode packages 208. Prisms 272 are orthogonal to the direction in whichlight is emitted from the light-emitting diodes. Prisms 272 may extendentirely across the light guide layer or may only extend partiallyacross the light guide layer. The density of prisms 272 may be uniformacross the light guide layer or may vary across the light guide layer(e.g., may vary along the X-axis and/or Y-axis).

Prisms 272 may have any desired shape. FIG. 16 shows an illustrativeexample of a cross-sectional shape for prisms 272. As shown in FIG. 16,optical film 70-1 may include a base film portion 270 with prisms 272extending from a lower surface 271 of the base film portion 270. Theprisms 272 in FIG. 16 may be formed from the same material as base filmportion 270 or a different material than base film portion 270. Eachprism 272 in FIG. 16 has a planar surface 274 that meets a curvedsurface 276 at a vertex 278. Planar surface 274 may be positioned eitheron the side of the prism 272 closer to the light-emitting diodes or theside of the prism further from the light-emitting diodes.

FIG. 17 shows an illustrative example of a cross-sectional shape forprisms 272. As shown in FIG. 17, optical film 70-1 may include a basefilm portion 270 with prisms 272 extending from a lower surface 271 ofthe base film portion 270. The prisms 272 in FIG. 17 may be formed fromthe same material as base film portion 270 or a different material thanbase film portion 270. Each prism 272 in FIG. 17 has a curved surface280 that meets a curved surface 282 at a vertex 284. In this example,the curvature of curved surfaces 280 and 282 is the same. However, thisexample is merely illustrative. In a similar example, shown in FIG. 18,the curvature of surface 280 is different than the curvature of surface282.

FIG. 19 shows an illustrative example of a cross-sectional shape forprisms 272. As shown in FIG. 19, optical film 70-1 may include a basefilm portion 270 with prisms 272 extending from a lower surface 271 ofthe base film portion 270. The prisms 272 in FIG. 19 may be formed fromthe same material as base film portion 270 or a different material thanbase film portion 270. Each prism 272 in FIG. 19 has a curved surface286 with a rounded tip 288 (as opposed to the vertices of FIGS. 16-18).

In general, each prism in turning film 70-1 may have any desiredstructure. The prisms may have different structures, the samestructures, structures that vary in any desired pattern, etc. The prismsmay themselves have a uniform cross-sectional shape or a cross-sectionalshape that varies along the length of the prism. The prisms may beformed on the upper surface of the light guide layer and/or the lowersurface of the light guide layer. Any of the potential prismarrangements described herein may be used in the light guide layer ofFIGS. 4 and 5.

Having the prisms of turning film 70-1 on the lower surface of theturning film and the light-scattering structures 252 of the light guidelayer on the upper surface of the light guide layer may help preventscratching of the layers and/or wetting between the layers. Additionalstructures (e.g., laser dots) may be included on the turning film (orother optical films in the electronic device) to prevent scratchingand/or wetting.

Light guide layer 78 (and corresponding light-scattering structures 252)may be used to control the viewing angle of the display within theXZ-plane. For example, as shown in FIG. 9 the design of thelight-scattering structures 252 may control how much the light is spreadwithin the XZ-plane when exiting the light guide layer. Turning film70-1 (and corresponding prisms 272) may be used to control the viewingangle of the display within the YZ-plane. For example, as shown in FIG.15 the design of the prisms 272 may control how much the light is spreadwithin the YZ-plane when exiting the turning film. Light guide layer 78and turning film 70-1 therefore may be used to provide ahigh-viewing-angle display with a high viewing-angle both along theX-axis and along the Y-axis.

It is generally desirable for homogenous backlight to be emitted frombacklight structures 42. It is also generally desirable for homogenouslight to be emitted from display layers 46 when desired. For example, ifall of the display pixels within the display are set to emit whitelight, it is desirable for the white light to be uniform across theentire display. Numerous steps may be taken to ensure uniform emissionof light from display 14 in electronic device 10.

FIG. 20 is a cross-sectional side view of backlight structures includingadditional ink or colored layers to correct color and/or luminancevariations in the emitted backlight. Similar to FIG. 4, FIG. 20 is across-sectional side view of backlight structures 42 taken along line 98in FIG. 3. In FIG. 20 additional ink layers that may be used to adjustcolor and/or luminance are shown.

First, adhesive layer 107 (sometimes referred to as light guide filmfixing tape) may include additional ink to control the color of theadhesive layer. Gray ink may be included in or on the adhesive layer 107to help ensure uniform luminance at the edge of the light guide layer.Alternatively, colored ink (e.g., ink that is blue, yellow, red, green,or any other desired color) may be included in or on the adhesive layer107 to ensure uniform color of light at the edge of the light guidelayer.

Other components near light guide layer 78 may have their color selectedto correct color and/or luminance variations in the emitted backlight ormay have an attached additional ink layer to correct color and/orluminance variations. For example, an additional ink layer 302 may beattached to the edge surface of light guide layer 78 or an additionalink layer 304 may be attached to the edge of chassis 102. The additionalink layers 302 and 304 may include gray ink or colored ink (e.g., inkthat is blue, yellow, red, green, or any other desired color). Theadditional ink layers may be, for example, layers of tape having aspecific desired color. Alternatively, ink or pigment may be added tochassis 102 itself such that the edge surface of the chassis has aspecific desired color. For example, gray ink or blue ink may be addedto the plastic chassis 102.

In another example, an additional ink layer 306 may be attached to thebottom surface of light guide layer 78 between reflector 80 and adhesivelayer 107. Alternatively, an additional ink layer 308 may be attached tothe upper surface of metal chassis 103 between reflector 80 and adhesivelayer 107. The additional ink layers 306 and 308 may include gray ink orcolored ink (e.g., ink that is blue, yellow, red, green, or any otherdesired color). The additional ink layers may be, for example, layers oftape having a specific desired color. Alternatively, ink or pigment maybe added to chassis 103 such that the upper surface of the chassis has aspecific desired color. For example, gray ink or blue ink may be addedto the metal chassis 103. Ink layers 302, 304, 306, and 308 may bereferred to as layers, coatings, ink layers, tape layers, colorcorrecting layers, compensating layers, luminance correcting layers, orpaint layers. Each layer (302, 304, 306, 308) does not necessarily needto be formed from ink. Each layer may be formed from ink, pigment, orany other material that creates a difference in color or luminancecompared with the surrounding material.

These examples are merely illustrative. Additional layers having anydesired color may be incorporated at any desired location within thedisplay to correct luminance or color issues if desired.

In some cases, pixel values may be reduced (truncated) in order tocorrect the white point of displayed light. Additionally, sometimesduring operation of the display, displayed content may need to rapidlymove across the display. For example, when a user is scrolling throughthe display the displayed content moves rapidly across the display. Toavoid visible artifacts when content is moving rapidly across thedisplay, voltages used in operating the display may be overdriven. Forexample, the data lines that provide data to the display pixels orswitching transistors involved in operating the display pixels may beoperated with an overdriven voltage (e.g., a voltage that is higher thanduring normal operation). Overdriving the display pixels in this way mayspeed up the time it takes to refresh the pixel, reducing visibleartifacts caused by transitions between brightness levels whendisplaying rapidly changing content.

When in the overdrive mode, pixels of a certain color may be overdrivenless than other pixels to avoid visible artifacts. For example, a greenshadow may be visible in situations in which content is moving rapidlyacross the display and truncation in red and blue pixels are high. Toprevent this type of visible artifact, the green pixels may beoverdriven less than the red and blue pixels (e.g., the overdrivevoltage of the green pixels will be lower than the overdrive voltage forthe red and blue pixels). In other words, pixels of different colors maybe overdriven by different amounts to ensure that no visible artifactsare present. This concept will be described in more detail in connectionwith FIGS. 43-47.

FIG. 21 is a top view of an illustrative light guide layer that may beincluded in backlight structures 42. As shown in FIG. 21, light guidelayer 78 may have tabs 78T. Each tab 78T may protrude from an edge ofthe light guide layer. Tabs 78T may therefore sometimes be referred toas protrusions 78T. Protrusions 78T may be used to secure the lightguide layer within the electronic device. For example, adhesive may beattached to an upper or lower surface of the protrusions.

In some cases, tabs 78T may cause visible artifacts when viewing thedisplay. For example, at a high viewing angle the areas of light guidelayer 78 adjacent to tabs 78T in the active area may appear darker thanother areas. To avoid this type of visible artifact, an additional layermay be included on the tabs to reduce the differences between the areasadjacent to the tabs and the areas not adjacent to the tabs.

FIG. 22 is a cross-sectional side view of backlight structures 42showing a tab 78T of light guide layer 78. As shown, adhesive layer 107may attach the tab to chassis 103. FIG. 22 shows an additional opticalfilm 70-2 positioned above optical film 70-1. Optical film 70-2 may be,for example, a diffuser layer, a brightness enhancement film, or anyother desired type of optical film. An opaque masking layer 122 may beformed on the lower surface of optical film 70-2. Opaque masking layer122 may be formed from black ink or another desired opaque material. Thepresence of the opaque masking layer may contribute to the contrastdifference in regions of the light guide layer adjacent to the tabs 78T.

Additional ink layer 124 may be included over tab 78T to correctcontrast differences in the tab region. The additional ink layer 124(sometimes referred to as coating 124, layer 124, tape layer 124, colorcorrecting layer 124, compensating layer 124, luminance correcting layer124, or paint layer 124) may have a color selected to correct contrastdifferences in the tab regions. The ink layer 124 may be black, gray, orany other desired color. Layer 124 may be formed only over the tab 78Tof light guide layer 78 or may be partially formed over non-tab portionsof light guide layer 78. The layer may overlap some of the active areaof the display or may not overlap the active area of the display. InFIG. 22, layer 124 is depicted as being formed on turning film 70-1(e.g., layer 124 directly contacts the upper surface of turning film70-1). This example is merely illustrative. If desired, the layer 124may be formed in direct contact with the light guide layer 78 (e.g., anupper surface of a tab 78T of the light guide layer). The layer 124 maybe interposed between light guide layer 78 and turning film 70-1.Similarly, opaque masking layer 122 may be formed at any desiredlocation within the display (e.g., on an upper or lower surface of anadditional optical film, between and in direct contact with two adjacentoptical films, etc.). Layers 122 and 124 may be formed from ink,pigment, or any other material that creates a difference in color orluminance compared with the surrounding material.

In some cases, a compensating layer may be used to modify the backlightprovided to display layers 46 to compensate for variations in displaylayers 46. For example, consider an example where display layers 46receive homogenous backlight with a uniform color and luminance and thedisplay layers are configured to display a uniform white color on theactive area of the display. Because the backlight has a uniform colorand luminance, the display layers would ideally emit a uniform whitelight. However, even if receiving backlight with a uniform color andluminance, variations in the display layers may cause the emitted lightto have a non-uniform appearance.

FIG. 23 is a top view of display layers 46 attempting to display auniform white color (when receiving uniform backlight). Despitereceiving uniform backlight, the display layers may have a band 310along an upper edge of the display that is brighter than the remainingportion 312 of the display. The bright band 310 may be caused byvariations within the display layers (e.g., thickness variations at theedge of the display layers). To correct for these types of a variationswithin the display layers, non-uniform backlight may deliberately beprovided to the display layers. For example, the backlight provided todisplay layers 46 in region 310 may be dimmer than the backlightprovided to display layers in region 312.

To dim the backlight provided to the display layers in region 310, acompensating layer may be included between the light guide layer 78 andthe display layers. FIG. 24 is a top view of a light guide layer showinghow compensating layer 314 may cover a portion of the light guide layer.The compensating layer 314 may have a length 318 and width 316. Becausethe non-uniformity of the display layers (in this example) is along theupper edge of the display, the compensating layer is also formed alongthe upper edge of the display layer.

Compensating layer 314 (sometimes referred to as coating 314, layer 314,tape layer 314, color correcting layer 314, compensating layer 314,luminance correcting layer 314, or paint layer 314) may have a colorselected to correct for non-uniformities in the display layers.Compensating layer 314 may be formed from ink, pigment, or any othermaterial that creates a difference in color or luminance compared withthe surrounding material. The compensating material may be formed at anydesired position between the light guide layer and the display layers(e.g., on the light guide layer, on an optical film, etc.). Thecompensating layer may be formed from gray ink or blank ink, in oneexample. The density of the ink may vary within the compensating layerif desired (e.g., the ink may have a higher density closer to the edgeof the light guide layer).

The shape of the region covered with compensating layer 314 in FIG. 24is merely illustrative. Compensating layer 314 may have any desiredshape. FIG. 25 shows an example where compensating layer 314 covers asmaller area of light guide layer 78 than in FIG. 24. In FIG. 25,compensating layer 314 has a length 322 that is less than the length 318of FIG. 24. Similarly, compensating layer 314 has a width 320 that isless than the width 316 of FIG. 24. Compensating layer 314 may havecurved edges and may extend only partially across the upper edge of thelight guide layer, as shown in FIG. 25.

The example in FIGS. 24 and 25 of compensating layer 314 covering theupper edge of the light guide layer is merely illustrative. In thisexample, the light-emitting diodes that provide light to the light guidelayer are positioned along the left edge of the display and thecompensating layer 314 is positioned along the upper edge of thedisplay. However, compensating layer 314 may be positioned along theleft edge of the display or any other desired location of the display.More than one compensation layer (optionally having different colors orother properties) may also be used to correct for variations within thedisplay layers.

FIG. 26 is a cross-sectional side view backlight structures that includea compensating layer to correct for variations in display layers 46. Asshown in FIG. 26, compensation layer 314 may also be formed on the lowersurface of an additional optical film 70-2 (e.g., a diffuser layer,brightness enhancement film, etc.). Compensation layer 314 may insteadbe formed on turning film 70-1 if desired. Compensation layer 314 may beformed at any other desired location within the display (e.g., directlyon the upper surface of light guide layer 78, on the upper surface of anadditional optical film, etc.).

As shown in the cross-sectional side view of FIG. 27, an additionaloptical film 70-2 may be incorporated between turning film 70-1 anddisplay layers 46. Optical film 70-2 may be a diffuser layer, abrightness enhancement film, a compensation film for enhancing off-axisviewing, a polarizer layer, a combination of one or more of these etc.FIG. 28 is a cross-sectional side view of optical film 70-2. As shown inFIG. 28, optical film 70-2 may be formed from a plurality of layers(e.g., layer 324-1, layer 324-2, layer 324-3, layer 324-4, and layer324-5). These layers may include pressure sensitive adhesive (PSA)layers, polarizing layers, and/or any other desired type of layer. Toimprove the defect hiding power of the optical film, the optical filmmay include a diffusive pressure sensitive adhesive layer. FIG. 29 is across-sectional side view of optical film 70-2 that includes diffusivepressure sensitive adhesive layer 324-6. In this example, optical film70-2 may be a brightness enhancement film. Including the diffusivepressure sensitive adhesive layer in the brightness enhancement film maycause additional diffusion of light received from the light guide layer.This may hide particle defects in the underlying layers (e.g., defectscaused by contaminant particles or other kinds of defects).

The example of diffusive pressure sensitive adhesive layer 324-6 beinginterposed between other layers in brightness enhancement film 70-2 ismerely illustrative. Diffusive pressure sensitive adhesive layer 324-6may instead be the upper-most layer in brightness enhancement film 70-2or the lowest layer in brightness enhancement film 70-2. In someembodiments, diffusive pressure sensitive adhesive layer 324-6 may beattached to and in direct contact with an additional pressure sensitiveadhesive layer (e.g., a pressure sensitive adhesive layer that does nothave diffusive properties). For example, layer 324-2 in FIG. 29 may be apressure sensitive adhesive layer (that does not have diffusiveproperties) or layer 324-3 in FIG. 29 may be a pressure sensitiveadhesive layer (that does not have diffusive properties).

During manufacturing, some electronic devices may be produced withdefects that affect the quality of the display. The defects may resultin artifacts that are visible if corrective action is not taken. Withoutcorrective action, some of the manufactured electronic devices may notbe suitable for use. To increase the number of manufactured devices thatmay be used, display compensation techniques may be applied to theelectronic devices.

FIG. 30 shows an illustrative system 326 for capturing images of adisplay and determining if compensation is required. As shown in FIG.30, an electronic device being tested (sometimes referred to as a deviceunder test) may emit light in direction 328 using display 14. The deviceunder test (DUT) may attempt to emit a uniform white image or some otherdesired test image. An image sensor 330 facing direction 332 may facethe display of the DUT. The image sensor may capture an image of thedisplay to determine if any irregularities are present in the display.The detected irregularities may be used to generate a pixel compensationtable that is used to correct for the irregularities during operation ofthe device. For example, image sensor 330 may detect that, whenattempting to display an entirely white image, a portion of the displayof the DUT is slightly blue. The pixels in the slightly blue region ofthe display may therefore be configured to show a slightly yellow image.The yellow tint of the display pixels compensates for the blue tendencyof the display, resulting in a uniform white image across the display.

FIG. 31 is a diagram of illustrative method steps that may be used togenerate compensation information for a display in an electronic device.At step 342, an image of the display in an electronic device may becaptured (e.g., using image sensor 330 in FIG. 30) while the displaydisplays a known image (e.g., all white). Next, at step 344, the imagedata may be grouped into representative blocks. For example, the imagesensor that captures the image in step 342 may have a given number ofrows and columns pixels. Each pixel may have a corresponding imagesignal. At step 344, the image signals from more than one imaging pixelmay be grouped (e.g., summed, averaged, or grouped using some othertechnique) into a single value that represents a block of pixels withinthe display.

Next, at step 346, a compensation gain table may be determined for eachtype of pixel in the display. For example, if the representative valuedetermined in step 344 indicates that the display has a bluish tint whenattempting to display white, compensation values for each pixel type(e.g., red pixels, blue pixels, and green pixels) may be determined thatresult in the display emitting a desired white color when attempting todisplay white. At step 346, the compensation gain table may have acorresponding compensation gain table for each color of pixel in thedisplay (e.g., gain tables for the red, blue, and green pixels for atotal of three gain tables). Each compensation gain table may have acompensation value for each block of pixels grouped in step 344.

Finally, at step 348, the compensation gain tables of step 346 may beextrapolated to determine a per-pixel compensation gain table for eachcolor of pixel in the display. During operation of the display, eachpixel may have its pixel value modified by the gain value from theper-pixel compensation gain table before the pixel displays the pixelvalue.

FIG. 32 shows a schematic diagram of illustrative circuitry that may beused in implementing display 14 of device 10. During operation ofelectronic device 10, control circuitry in the device may supply imagedata 356 for images to be displayed on display 14. Ultimately, the imagedata may be delivered to display driver circuitry 354, which may supplythe image data to data lines of the display. Display driver circuitry354 may also include gate driver circuitry which is used to assert gateline signals on gate lines of display 14. The display driver circuitrymay be used to provide the image data to pixels 90 in the display.

Before being provided to display driver circuitry 354, the image datamay be multiplied by compensation factors from gain table 358 inmultiplication circuit 352 (sometimes referred to herein as a gaincircuit or modification circuit). Each frame of image data 356 mayinclude a representative brightness value for each pixel 90. Gain table358 may include a compensation factor for each pixel 90. Thecompensation factor may correct for display uniformity issues (asdetermined using the method of FIG. 31, for example). For example, ifwhile performing the method of FIG. 31 it is determined that the displayhas a bluish tint when attempting to display white, the brightness of ablue pixel in this region may be reduced to ensure a white color isdisplayed when desired.

After image data 356 is multiplied by the dimming factors from gaintable 358, the modified image data may be provided to display drivercircuitry 354. Display driver circuitry 354 will then provide themodified image data to the pixels in the display. The pixels may thendisplay the desired image.

Multiplication circuit 352, gain table 358, display driver circuitry354, and pixels 90 as shown in FIG. 32 may sometimes be collectivelyreferred to as display circuitry. Alternatively, pixels 90 may sometimesbe referred to as a display while multiplication circuit 352, gain table358, and display driver circuitry 354 may sometimes collectively bereferred to as control circuitry. The example of a multiplicationcircuit 352 that multiplies image data 356 by compensation factors fromper-pixel gain table 358 is merely illustrative. Other desired types ofmodifications (in addition or instead of multiplication) may be used tomodify image data 356 based on compensation factors 358. For example,the per-pixel gain table may include compensation factors that are addedto the image data to produce the modified image data. Circuit 352 maytherefore sometimes be referred to as an image data modificationcircuit.

In FIG. 3, it was discussed how information to be displayed on display14 may be conveyed to a display driver integrated circuit using a signalpath such as a signal path formed from conductive metal traces in arigid or flexible printed circuit such as printed circuit 64 (as anexample). FIG. 33 is a cross-sectional side view of a flexible printedcircuit (64) that is used to convey signals from a rigid printed circuitboard to a display driver integrated circuit 402 and from the displaydriver integrated circuit 402 to thin-film transistor layer 58. Thedisplay driver integrated circuit may be mounted directly to flexibleprinted circuit 64. This may be referred to as a chip-on-flex (COF)arrangement. The flexible printed circuit may be coupled betweenthin-film transistor layer 58 and printed circuit board 404. Printedcircuit board 404 may be, for example, a rigid printed circuit board(sometimes referred to as a motherboard).

In region 406, flexible printed circuit 64 is coupled to thin-filmtransistor (TFT) layer 58. In region 408, flexible printed circuit 64 iscoupled to rigid printed circuit board 404. In region 406, a conductivelayer 412 may couple a contact 410 of the flexible printed circuit to acontact 414 of the thin-film transistor layer. Contacts 410 and 414 maybe contact pads, traces, or any other desired conductive layer.Conductive layer 412 may be anisotropic conductive film (ACF), may besolder, or may be formed from any other desired conductive material. Inregion 408, a conductive layer 418 may couple a contact 416 of theflexible printed circuit to a contact 420 of the rigid printed circuitboard. Contacts 416 and 420 may be contact pads, traces, or any otherdesired conductive layer. Conductive layer 418 may be anisotropicconductive film (ACF), may be solder, or may be formed from any otherdesired conductive material. Flexible printed circuit 64 may have a bentregion 407 interposed between regions 406 and 408.

To add structural support for display driver integrated circuit 402, astiffener 422 may be included on flexible printed circuit 64. Stiffener422 may be formed on an opposing side of flexible printed circuit 64 asdisplay driver integrated circuit 402. The presence of stiffener 422protects integrated circuit 402 from vibrations or drop events that mayotherwise damage the integrated circuit. Stiffener 422 may be formedfrom stainless steel or another desired material. Stainless steel may bestiff enough to provide sufficient structural support for the integratedcircuit 402.

FIG. 34 is a top view of flexible printed circuit 64 in an unbent state.As shown, stainless steel stiffener 422 may extend along the length ofthe flexible printed circuit. The stiffener has a first portion (on theside of attachment region 408) with a first length and a second portion(on the side of bending region 407) with a second length that isdifferent than the first length. The total length of the stiffener maybe more than three times greater than its width, more than five timesgreater than its width, more than seven times greater than its width,less than twelve times greater than its width, etc. This example ismerely illustrative. In general, stiffener 422 may have any desiredshape. The flexible printed circuit may extend along an edge of thethin-film transistor layer. In one example, the flexible printed circuitmay extend along the same edge of the thin-film transistor layer thatoverlaps the light-emitting diodes of the backlight structures (e.g.,the flexible printed circuit may extend along the left edge of thethin-film transistor layer). This example is merely illustrative, andthe flexible printed circuit (and stiffener) may extend along anydesired edge of the thin-film transistor layer.

FIG. 35 is a cross-sectional side view of attachment region 408 of theflexible printed circuit showing how flexible printed circuit 64 isattached to rigid printed circuit board 404. FIG. 35 shows how printedcircuit board 404 includes a conductive layer 420 that is coupled toanisotropic conductive film 418. The anisotropic conductive film is alsocoupled to trace 416 of flexible printed circuit 64.

As shown in FIG. 35, a portion 432 of trace 416 may not directly contactanisotropic conductive film 418. Portion 432 of trace 416 is thereforeseparated from the upper surface of rigid printed circuit board 404. Toimprove reliability of the trace, a solder mask layer 434 may beincluded between portion 432 of trace 416 and rigid printed circuitboard 404. Without solder mask layer 434 present, a gap 438 may bepresent between trace 416 and the upper surface of rigid printed circuitboard 404. With this large gap, traces 416 may dent or crack afterbonding (due to compression on portion 432 during the bonding processand portion 432 bending towards rigid printed circuit board 404). Toprevent these issues, solder mask layer 434 may be included belowportion 432 of trace 416. With the presence of solder mask layer 434,the gap 436 below portion 432 of trace 416 is much smaller (e.g., thangap 438). Reducing the size of the gap below portion 432 of trace 416improves reliability of the trace by preventing portion 432 from bendingand cracking.

Gap 436 may be less than 1 millimeter, less than 0.1 millimeters, lessthan 0.01 millimeters, less than 0.001 millimeters, etc. In some cases,the gap may be removed entirely (and portion 432 of trace 416 directlycontacts solder mask layer 434). The example of including solder masklayer 434 below portion 432 of trace 416 is merely illustrative. Ingeneral, any desired type of material may be incorporated below portion432 of trace 416 to prevent damage to the trace. Solder mask layer 434may therefore instead sometimes be referred to as a layer, dielectriclayer, gap-filling layer, filler, etc. Using the solder mask material asthe gap-filling layer may be advantageous for manufacturing as thesolder mask material may already be deposited on the rigid printedcircuit board during the manufacturing process. Therefore, no additionalmanufacturing steps are required to use the solder mask material asgap-filling layer 434.

When flexible printed circuit 64 is bonded to rigid printed circuitboard 404 in region 408, excess material that forms anisotropicconductive film (ACF) 418 may flow over the edge of the printed circuitboard. This process may be referred to as squeeze-out, for example. Ifgap-filling layer 434 extended as a strip across the entire printedcircuit board 404, the paths for the excess ACF material may be blocked.This may result in poor anisotropic conductive film compression (and apoor electrical connection between the flexible printed circuit andrigid printed circuit board).

To fill the gap between trace 416 and rigid printed circuit board 404while still providing channels for excess ACF material to flow duringcompression, a number of discrete gap-filling layers 434 may be formedalong the edge of the rigid printed circuit board. FIG. 36 is a top viewof the rigid printed circuit board showing a number of discretegap-filling layers 434 along the edge of rigid printed circuit board404. Each gap-filling layer 434 may be completely laterally surroundedby portions of rigid printed circuit board 404 that are not attached tothe gap-filling layer. The discrete gap-filling layers 434 may bereferred to as island-type or island-shaped gap-filling layers.

Returning to FIG. 35, encapsulant such as encapsulant 442 and 444 may beincorporated in attachment region 408. The encapsulant may providestrain relief to improve reliability and robustness of the attachmentbetween flexible printed circuit 64 and rigid printed circuit board 404.

Encapsulant 444 is attached to a lower surface of flexible printedcircuit 64 and an edge surface of rigid printed circuit board 404.Encapsulant 444 may protect exposed metal traces on flexible printedcircuit 64 from touching the edge of printed circuit board 404.Encapsulant 444 may also absorb heat generated by the electronic device.Encapsulant 444 may also prevent moisture or other contaminants fromreaching the conductive components that form the attachment betweenflexible printed circuit 64 and rigid printed circuit board 404.Encapsulant 444 may conform to the edge of printed circuit board 404 ifdesired.

Encapsulant 442 is attached to an upper surface of flexible printedcircuit 64, an edge surface of flexible printed circuit 64, and an uppersurface of rigid printed circuit board 404. Encapsulant 442 may alsoabsorb heat generated by the electronic device. Encapsulant 442 may alsoprevent moisture or other contaminants from reaching the conductivecomponents that form the attachment between flexible printed circuit 64and rigid printed circuit board 404. Encapsulant 442 may be formed froma conformal material that conforms to the edge of the flexible printedcircuit the edge of anisotropic conductive film 418, and the edges oftraces 416 and 420. Encapsulant 442 and 444 may be formed from the samematerial or different materials. The encapsulation may be formed from anelastic material (e.g., a material with a low Young's modulus).

FIG. 37 is a top view of flexible printed circuit 64 in an unbent stateshowing how the pitch of traces 410 may vary across the flexible printedcircuit. As shown, traces 410 may be distributed across the flexibleprinted circuit in attachment region 406 (e.g., that attaches theflexible printed circuit to thin-film transistor layer 58). In general,it is desirable to position the traces close together (as this may allowmore traces to be included on the flexible printed circuit in the sameamount of area). However, the traces must be separated by a sufficientdistance to ensure that the traces do not touch (thus shorting thetraces and preventing them from functioning correctly).

Based on the manufacturing methods in producing flexible printed circuit64, the tolerance in producing the traces may vary dependent upon theposition of the trace within the flexible printed circuit. For example,the position of traces in the center of the flexible printed circuit maybe more controllable (e.g., have a lower tolerance) than the traces atthe edges of the flexible printed circuit. Therefore, the pitch of thetraces may vary across the flexible printed circuit to match themanufacturing tolerance.

As shown in FIG. 37, traces 410-1 in the center of the flexible printedcircuit may have a corresponding pitch 452-1. Because manufacturingtolerance is smallest in the center of the flexible printed circuit (andtherefore the position of the traces is best controlled in the center ofthe flexible printed circuit), pitch 452-1 may be small. Traces 410-2are closer to the edge of the flexible printed circuit than traces410-1. The manufacturing tolerance for traces 410-2 may be larger thanthe manufacturing tolerance for traces 410-1. Therefore, the position oftraces 410-2 cannot be controlled as well as the position of traces410-1. Accordingly, pitch 452-2 is greater than pitch 452-1.Manufacturing tolerance may be at a maximum at the edges of the flexibleprinted circuit (and therefore the position of the traces is worstcontrolled in the edges of the flexible printed circuit). Therefore,pitch 452-3 may be larger than pitches 452-2 and 452-1.

FIG. 38 is a graph showing pitch and density of traces 410 as a functionof position along the flexible printed circuit. As shown, the traces mayhave a pitch that follows an illustrative profile 454. At the edges ofthe flexible printed circuit, the pitch is at its highest levels. At thecenter of the flexible printed circuit, the pitch is at its minimumlevel. The traces may have a uniform width. Therefore, the density ofthe traces is inversely proportional to the pitch of the traces. Profile456 shows an illustrative profile for the density of the traces. At theedges of the flexible printed circuit, the density is at its lowestlevels. At the center of the flexible printed circuit, the density is atits highest level. The illustrative profiles shown in FIG. 38 are merelyillustrative. The profiles may have sloped portions (as in FIG. 38) ormay follow a step-function that follows a similar shape as profiles 454and 456.

The pitch and density profiles may have a similar shape as a profile ofthe manufacturing tolerance of the traces. FIG. 39 is a graph showingthe pitch profile of the traces relative to the tolerance profile forthe traces. Profile 458 shows the tolerance of the traces as a functionof position within the flexible printed circuit. As shown, the toleranceincreases as the traces get further from the center of the flexibleprinted circuit and closer to the edges of the flexible printed circuit.

Profile 460 shows a profile for traces having a constant pitch acrossthe flexible printed circuit. As shown, when the pitch of the tracesdoes not change, margin 464 between the pitch and the tolerances at thecenter of the flexible printed circuit may be larger than margin 466between the pitch and the tolerances at the edges of the flexibleprinted circuit. At the center of the flexible printed circuit, margin464 may be too large (meaning that a lower-than-necessary number oftraces are fit at the center of the flexible printed circuit). At theedges of the flexible printed circuit, margin 466 may be small (meaningthat the traces may be susceptible to being shorted together).

To avoid these issues and have a consistent margin across the flexibleprinted circuit, the pitch may instead vary across the flexible printedcircuit as shown by profile 462. In profile 462, the pitch increasesstepwise from a minimum at the center of the flexible printed circuit toa maximum at the edge of the flexible printed circuit. This allows thepitch to follow the same profile as tolerance profile 458 and ensuresthat the margin remains consistent regardless of position (e.g., margin468 close to the center is the same as margin 470 close to the edge).

The examples of profile shapes in FIGS. 38 and 39 are merelyillustrative. In general, the pitch of the traces may follow any desiredprofile. For example, the pitch profiles may be asymmetrical (instead ofsymmetrical as in FIG. 38). If manufacturing tolerance was insteadhighest in the center of the flexible printed circuit, the pitch may behighest in the center of the flexible printed circuit as well. The pitchof the traces at any given position may be based on the manufacturingtolerance for the traces at that position.

FIG. 40 is a cross-sectional side view of flexible printed circuit 64and thin-film transistor layer 58 in attachment region 406. FIG. 40shows how thin-film transistor layer 58 includes a conductive layer 414that is coupled to anisotropic conductive film 412. The anisotropicconductive film is also coupled to trace 410 of flexible printed circuit64. To prevent damage to traces on flexible printed circuit layer 64(e.g., caused by flexible printed circuit 64 contacting the edge ofthin-film transistor layer 58 when bent), a protective layer 472 may beincluded. Protective layer 472 overlaps the edge of thin-film transistorlayer 58. The protective layer may be formed from a portion of a solderresist layer (e.g., a solder mask layer) or any other desired dielectricmaterial. Protective layer 472 may directly contact the edge ofthin-film transistor layer 58 when flexible printed circuit 64 is bent.

The traces of flexible printed circuit 64 may include both copper andtin. However, areas of the flexible printed circuit with tin may be morebrittle and may crack if bent (due to metallic bonding stresses).Therefore, tin may be omitted from the flexible printed circuit in thebent region of the flexible printed circuit board. FIG. 41 is across-sectional side view of an illustrative flexible printed circuitboard. As shown, the flexible printed circuit board may include a baselayer 482 (sometimes referred to as substrate layer 482 or polyimidelayer 482). The base layer 482 may be formed from any desired dielectricmaterial (e.g., polyimide). A copper trace 484 may be formed over thebase layer and a tin trace may be formed over the copper trace. Toprevent the tin trace from being present in bending region 407, a solderresist 490 (sometimes referred to as masking layer 490, solder mask 490,etc.) may be included in bending region 407. An additional solder resist488 may be included over the tin layer 486 (in the non-bending regions)and over the solder resist 490 (in bending region 407).

The arrangement of FIG. 41 may ensure that no tin is present in bendingregion 407. However, the presence of two solder resist layers (e.g.,solder resist 488 and solder resist 490) may result in a higherstiffness than desired in bending region 407. To ensure that no tin ispresent in bending region 407 and reduce stiffness in the bendingregion, an arrangement of the type shown in FIG. 42 may be used.

As shown in FIG. 42, a removable masking layer may be used to omit tinin bending region 407. Solder resist 488 may then fill the gap betweentin 486 and cover tin 486 in regions where the tin is present. In otherwords, solder resist 488 may directly contact both the upper surface ofcopper trace 484 (e.g., in bending region 407) and tin trace 486 (e.g.,outside of bending region 407). Removing the first masking layer used toomit tin in bending region 407 reduces the thickness of the flexibleprinted circuit board in bending region 407. The reduced thickness inbending region 407 reduces stiffness compared to FIG. 41.

Display 14 may be characterized by color performance statistics such aswhite point. The white point of a given display is commonly defined by aset of chromaticity values that represent the color produced by thedisplay when the display is generating all available display colors atfull power. Prior to any corrections during calibration, the white pointof the display may be referred to as the “native white point” of thatdisplay. For example, the native white point may be associated withoperating all of the red, green, and blue subpixels in the display atRGB levels of 255 (on a scale from 0-255).

Due to manufacturing differences between displays, the native whitepoint of a display may differ, prior to calibration of the display, fromthe desired (target) white point of the display. The target white pointmay be defined by a set of chromaticity values associated with areference white (e.g., a white produced by a standard display, a whiteassociated with a standard illuminant such as the D65 illuminant of theInternational Commission on Illumination (CIE), a white produced at thecenter of a display). In general, any suitable white point may be usedas a target white point for the display.

In some cases, pixel values may be reduced (truncated) in order tocorrect the white point of displayed light. For example, during displaycalibration, RGB values may be determined that correspond to variousreference white points (e.g., a first set of RGB values may beidentified that correspond to the D65 illuminant of the InternationalCommission on Illumination (CIE), a second set of RGB values may beidentified that correspond to the D110 illuminant of the CIE, etc.).FIGS. 43A and 43B shows illustrative tables of RGB values associatedwith different white points. FIG. 43A shows RGB values (sometimesreferred to as gray levels) of 255 for red, 255 for green, and 255 forblue (e.g., (255, 255, 255)). This may correspond to the native whitepoint of the display. FIG. 43B, meanwhile, shows RGB values resulting ina white point that matches the D65 illuminant of the InternationalCommission on Illumination (CIE). As shown in FIG. 43B, to produce thedesired white point, red has a gray level of 241, green has a gray levelof 254, and blue has a gray level of 234. In other words, the nativewhite point of the display may, in this example, be more blue than thetarget D65 illuminant. Therefore, the blue value is reduced (truncated)to match the D65 illuminant.

Truncating the gray levels of some of the subpixels allows for thedisplay to match a desired white point but may present other challenges.In particular, the length of time to update each subpixel between framesmay vary. For example, consider a scenario in which a user is scrollingthrough content on the display. In this use-case, the displayed contentmoves rapidly across the display. In one common scenario, the displaymay present content with a white background and black text. As the userscrolls, the black text moves across the white background. In thesetypes of scenarios, the pixels may frequently transition from white toblack.

As shown in FIGS. 43A and 43B, the truncated pixel values used toprovide the desired white point may result in differences in thewhite-to-black transitions of the different colored subpixels.White-to-black transition time may refer to the length of time it takesfor the pixel to transition from its white point gray level to within10% its black gray level (e.g., 0). As depicted in FIG. 43A, when thedisplay has gray level values of 255 for red, green, and blue subpixels,the white-to-black transition time of each pixel-type may be the same(e.g., the red pixel transition time is 10.0 seconds, the green pixeltransition time is 10.0 seconds, and the blue pixel transition time is10.0 seconds). It should be understood that these times are merelyillustrative. There may be some variance (e.g., ±0.1 milliseconds, ±0.2milliseconds, ±0.3 milliseconds, etc.) between the transition times evenwhen a gray level of 255 is used for each color pixel. However, becauseeach pixel is changing from a gray level of 255 to a gray level of 0,the transition times are generally similar.

As shown in FIG. 43B, when the truncated subpixel values are used (for acorrected white point), the white-to-black transitions of the differentcolored subpixels may vary. In FIG. 43B, the red subpixel transitiontime (e.g., from 241 to 0) is 9.5 seconds, the green subpixel transitiontime (e.g., from 254 to 0) is 10.0 seconds, and the blue subpixeltransition time (e.g., from 234 to 0) is 9.4 seconds. As a result ofthese differences in transition time, the display may have a greenishhue during white-to-black transitions (because the green brightnesslevel falls more slowly than the red and blue brightness levels).

FIG. 44 is a graph showing how the green brightness level may fallslowly during white-to-black transitions, resulting in a greenish hue onthe display during white-to-black transitions. A gray level responsecurve G is depicted for a green subpixel, a gray level response curve Ris depicted for a red subpixel, and gray level response curve B isdepicted for a blue subpixel. As shown, at t₀ the pixel may have RGBvalues corresponding to the white point of FIG. 43B for matching the D65illuminant. In other words, at t₀, the red subpixel has a value of 241,the green subpixel has a value of 254, and the blue subpixel has a valueof 234. At t₁, the pixel may begin to transition to black. Ultimately,at t₂, the red, blue, and green subpixels will all have a gray level of0. However, as shown in FIG. 44, the green pixel has a higher startingpoint and therefore has a response curve that is generally higher thanthe response curves of the red and blue pixels. The shapes of theresponse curves shown in FIG. 44 are merely illustrative. In general,each of the response curves may have any desired shape. However, thegreen response curve may be higher than the blue and red response curvesat one or more points during the transition, contributing to theapparent green hue during white-to-black transitions.

FIG. 45 illustrates how pixels may be provided with intermediate targetvalues to suppress color motion blur (e.g., an undesirable greenappearance during a white-to-black transition). Initially, a pixel inimage frame F1 may have red, green, and blue subpixel values 502 of(241, 254, 234) corresponding to the white point of FIG. 43B. Then, thepixels may need to transition from white to black (e.g., duringscrolling of a black text on a white background). Therefore, the finaltarget values for the red, green, and blue subpixels are (0, 0, 0). Thistarget pixel state 506 will be reached when display 14 displays frameF3.

However, to suppress the aforementioned green shadow, an intermediateset of target values is temporarily imposed on the pixels. The temporarytarget values for these pixel include increased (overdriven) red andblue subpixel values. In the example of FIG. 45, intermediate frame F2has been provided with temporary target values 504 for the red, green,and blue subpixels of (20, 0, 20)—i.e., the red subpixel value has beentemporarily increased and the blue subpixel value has been temporarilyincreased. Temporarily increasing the red and blue subpixel values slowsdown the red and blue transition times to more closely match the greentransition time. This therefore mitigates the undesired green appearanceduring white-to-black transitions.

The values provided in FIG. 45 are merely illustrative. In general, anydesired intermediate values may be used to mitigate artifacts associatedwith white-to-black transitions or other transitions in the display.When a pixel transitions from white to black, the red and blue subpixelsmay be temporarily overdriven (e.g., operated at a gray level that ishigher than the ultimate target gray level) while the green subpixelsmay not be overdriven or may be overdriven to a lesser degree than thered and blue subpixels (e.g., operated at a gray level that is at orclose to the ultimate target gray level).

A diagram of illustrative resources that may be used in device 10 toreduce color-artifacts such as the aforementioned green appearanceduring white-to-black transitions is shown in FIG. 46. As shown in FIG.46, device 10 may include control circuitry such as control circuitry508. Control may be part of control circuitry 16 in FIG. 1 if desired.Control circuitry 508 may include storage such as hard disk drivestorage, nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory configured to form a solidstate drive), volatile memory (e.g., static or dynamicrandom-access-memory), etc. Processing circuitry in control circuitry508 may be used to control the operation of device 10. This processingcircuitry may be based on one or more microprocessors, microcontrollers,digital signal processors, application specific integrated circuits,etc.

Control circuitry 508 may be used to run software on device 10, such asoperating system software, application software, firmware, etc. As shownin FIG. 46, the software running on control circuitry 508 may includecode that generates content that is to be presented on display 14 (see,e.g., content generator 510, which may be an operating system function,an e-book reader or other software application, or other code that isrunning on control circuitry 508). Content generator 510 may generatecontent that has not been corrected to reduce color-based transitionartifacts (uncorrected content) and this content may be supplied tographics processing unit 512.

Graphics processing unit 512 may include an input frame buffer such asbuffer 514 or other storage to maintain information on a current imageframe 516 and one or more earlier frames such as previous image frame518. Graphics processing unit 512 may also include an output framebuffer such as output frame buffer 524 that stores content in which oneor more pixels may have been modified to reduce transition artifactssuch as a green hue on white-to-black transitions. Content processor 520may use one or more look-up tables 522 to process uncorrected contentand produce corresponding updated content in which pixels have beenmodified to mitigate transition based artifacts.

Look-up tables may have associated overdrive gray levels associated witha starting gray level and an ending gray level. For example, in theexample provided in FIG. 45, a blue subpixel transitions from an initialvalue of 234 to a target value of 0. The look-up table may have an entryassociated with this scenario for a corresponding modified value of 20,as is reflected in FIG. 45. Any desired number of look-up tables may beused to generated the modified image data that is provided to outputframe buffer 524.

The updated content with decreased transition artifacts may be suppliedto display driver circuitry 526 of display 14. Display driver circuitry526 may include integrated circuit(s) and/or thin-film transistorcircuitry on display 14 for displaying the content that is received onthe pixels of the display.

Illustrative operations involved in using resources of the type shown inFIG. 46 in displaying content with reduced artifacts are shown in FIG.47. Initially, content generator 510 may generate content to bedisplayed on display 14 and graphics processing unit 512 may receive thecontent. The content may include frames of image data. Content processor520, which may be implemented using software and/or hardware resourcesassociated with graphics processing unit 512, may receive a frame ofimage data (sometimes referred to as an image frame or content frame)from content generator 510 at step 532.

During the operations of step 534, content processor 534 may use framebuffer 514 to store frames of image data including current frame 516 andprevious frame 518. Content processor 520 may compare the pixel valuesin current frame 516 and previous frame 518 to identify gray leveltransitions that may be susceptible to artifacts (e.g., an undesiredgreen hue). After each current frame is processed, processor 520 maystore the data of the current frame as previous frame 518.

At step 536, content processor 520 may use one or more look-up tables522 to determine intermediate values for one or more pixels. Theintermediate values may be used to mitigate artifacts associated withwhite-to-black transitions, for example. Frames of data that have beenprocessed by content processor 520 may be stored in output frame buffer524. The modified frame (i.e., a frame such as frame F2 of FIG. 45) canbe output at step 538 and subsequently displayed by display drivercircuitry 526. As indicated by line 540, processing can then loop backto step 532 so that additional content from content generator 510 can beprocessed.

If desired, the image processing operations involved in implementing thetransition artifact mitigation process of processor 520 may beimplemented in full or in part in control circuitry 508 (e.g., as partof an operating system or application or both an operating system andapplication), may be implemented in full or in part in display 14 (e.g.,using resources in a timing controller integrated circuit or othercircuitry in display drier circuitry 526), may be implemented in full orin part on graphics processing unit 512 as described in connection withFIG. 46, and/or may be implemented using other resources in device 10 orany combination of two or three or more of these sets of resources. Theuse of a scenario in which blur abatement image processor 520 isimplemented on graphics processing unit 512 is merely illustrative.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An electronic device comprising: display layers;and a backlight unit that provides backlight for the display layers, thebacklight unit comprising: a light guide film; a plurality oflight-emitting diode packages, wherein each light-emitting diode packageincludes two or more light-emitting diodes connected in series that emitlight into an edge of the light guide film; and a flexible printedcircuit board, wherein each light-emitting diode package is flip chipbonded to the flexible printed circuit board and wherein a distancebetween each pair of adjacent light-emitting diode packages is less than0.5 millimeters.
 2. The electronic device defined in claim 1, whereineach light-emitting diode package is flip chip bonded to the flexibleprinted circuit board with first, second, and third flip chip bumps. 3.The electronic device defined in claim 2, wherein each light-emittingdiode package has a first recess that receives the first flip chip bump,a second recess that receives the second flip chip bump, and a thirdrecess that receives the third flip chip bump.
 4. The electronic devicedefined in claim 3, wherein the second recess is interposed between thefirst and third recesses and wherein the second recess has a smallerwidth than the first and third recesses.
 5. The electronic devicedefined in claim 1, further comprising: a chassis that supports thelight guide film; and an adhesive layer that attaches the light guidefilm to the chassis, wherein the adhesive layer includes an opticalcomponent that alters one of the color and luminance of the backlight.6. The electronic device defined in claim 5, further comprising: areflector layer on a lower surface of the light guide film; and acompensation layer on the lower surface of the light guide film betweenthe reflector layer and the adhesive layer.
 7. The electronic devicedefined in claim 5, further comprising: a reflector layer on a lowersurface of the light guide film; and a compensation layer on the chassisbetween the reflector layer and the adhesive layer.
 8. The electronicdevice defined in claim 5, wherein the light guide film has anadditional edge opposite the edge of the light guide film, theelectronic device further comprising: a compensation layer on theadditional edge of the light guide film.
 9. The electronic devicedefined in claim 5, wherein the chassis is a metal chassis, theelectronic device further comprising: a plastic chassis attached to themetal chassis; and a compensation layer on the plastic chassis.
 10. Theelectronic device defined in claim 1, wherein the light guide film has aplurality of protrusions, the electronic device further comprising: acompensating layer formed over each protrusion of the plurality ofprotrusions.
 11. The electronic device defined in claim 1, furthercomprising: a compensating layer formed over the light guide film thatmodifies the backlight to compensate for a non-uniformity in the displaylayers.
 12. The electronic device defined in claim 11, wherein the edgeof the light guide film that receives light from the light-emittingdiodes is a left edge of the light guide film and wherein thecompensating layer comprises gray ink along an upper edge of the lightguide film.
 13. An electronic device comprising: display layers; and abacklight unit that provides backlight for the display layers, thebacklight unit comprising: a flexible printed circuit; a light guidefilm; a plurality of light-emitting diodes mounted on the flexibleprinted circuit that emit light into an edge of the light guide film; anoptical film that is interposed between the light guide film and thedisplay layers; and a compensating layer formed on the optical film,wherein the compensating layer modifies a portion of the backlight tocompensate for a non-uniformity in the display layers.
 14. Theelectronic device defined in claim 13, wherein the compensating layercovers some but not all of the light guide film.
 15. The electronicdevice defined in claim 13, wherein the edge of the light guide filmthat receives light from the light-emitting diodes is a left edge of thelight guide film and wherein the compensating layer is formed over onlyan upper edge of the light guide film.
 16. The electronic device definedin claim 13, wherein the compensating layer comprises gray ink formed onthe optical film.
 17. The electronic device defined in claim 13, whereinthe optical film is a diffuser layer.
 18. The electronic device definedin claim 13, wherein the optical film is a brightness enhancement film.19. An electronic device comprising: display layers; and a backlightunit that provides backlight for the display layers, the backlight unitcomprising: a flexible printed circuit; a light guide film having edgesand a plurality of tabs that protrude from the edges; a plurality oflight-emitting diodes mounted on the flexible printed circuit that emitlight into one of the edges of the light guide film; and a plurality ofcompensating layers that each alter at least one of the color andluminance of the backlight, wherein each one of the plurality ofcompensating layers is formed over a respective tab of the plurality oftabs.
 20. The electronic device defined in claim 19, wherein each one ofthe plurality of compensating layers comprises ink.